You're absolutely right. I had elided the details of the locking from my original use case, but now I look at it again, I see that even what I did have wasn't enough.

I've made a new patch that takes a lock properly. A non-blocking acquisition of the lock will serve the same purpose as looking to see if there is a line being read, and will also prevent the interpreter from starting to read a new line while the text is being printed.

My use case now looks like this.

reading_lock = threading.Lock ()
readline.set_lock (reading_lock)

def describe (indicator):
	if reading_lock.acquire (False):
		print (indicator.description)
		reading_lock.release ()
	else:
		print ()
		print (indicator.description)
		readline.forced_update_display ()